1. Field of the Invention
The present invention is generally related to a hydraulic actuator which can withstand a shock load and automatically return to its original position and, more particularly, to a hydraulic actuator used as a trim and tilt cylinder for a marine drive system, such as an outboard motor.
2. Description of the Prior Art
Those skilled in the art of hydraulic actuators are familiar with trim cylinders that are capable of withstanding a shock load and subsequently returning to the original position before the shock load occurred.
U.S. Pat. Nos. 3,999,502 and 4,050,359, which issued to Mayer in 1976 and 1977, respectively, disclose a hydraulic power trim and power tilt system for a combined power trim and shock absorbing piston-cylinder unit of an outboard motor. The system includes a reversal pump means having a trim-up port connected by pressure responsive pilot valve and a trim-down port through a reverse lock solenoid valve. A down-pilot spool valve provides full drain flow for trim-up and power flow for trim-down. An "up-reverse" pilot valve with a pressure operator is in parallel with the reverse lock valve and provides a restricted by-pass for limited trim-up in reverse. The trim-up hydraulic input or powered side of the cylinder units defines a trapped hydraulic system creating "memory" in the system so that after an impact occurs the motor can return to the original trim position. The return side permits relatively free-flow to permit "trail-out" under low impact. At high speed impact, the flow is restricted and the and the pressure within the cylinder increases. At a selected point, a shock valve within the piston-cylinder opens and absorbs the shock forces. The piston unit includes an inner floating head which is telescoped into a head secured to the piston rod with a chamber thereby formed to store the liquid flow during shock movement. A metered orifice and check valve allows return to the original trim-set position.
U.S. Pat. No. 4,493,659, which issued to Iwashita on Jan. 15, 1985, describes a tilt-lock mechanism. The mechanism is intended for vertically tiltable outboard stem drive units on a marine vessel and preferably includes a hydraulic cylinder device dampingly connecting the drive unit and the vessel. The mechanism preferably includes apparatus allowing the operator to selectively prevent or allow the drive unit to freely tilt under various operating conditions.
U.S. Pat. No. 5,392,690, which issued to Hundertmark on Feb. 28, 1995, describes a marine power steering system. The system is intended for operation of a power steering assembly includes a pressure accumulator to provide pressurized hydraulic fluid and valving that permits the transfer of hydraulic fluid within the cylinder to provide efficient use of hydraulic fluid.
U.S. Pat. No. 4,784,625, which issued to Nakahama on Nov. 15, 1988, describes a tilt lock mechanism for a marine propulsion device. The tilt locking and shock absorbing arrangement for a marine outboard drive embodies a single cylinder and piston assembly for controlling the tilt and trim positions of the drive and for further absorbing shocks applied to it. An accumulator arrangement is provided which has gas over oil and is valved into the system in such a way as to retard passage of the gas into the shock absorbing device per se and which will return any escaped gas back to the accumulator.
U.S. Pat. No. 5,584,225, which issued to Arvidsson et al on Dec. 17, 1996, describes a hydraulic cylinder, especially a trim and tip cylinder for outboard type boat propeller drive units. The hydraulic cylinder is provided with a piston that has axial channels with check valve permitting fluid flow in one direction from one cylinder chamber to the other. Constricting means in the form of a spring-biased plate is displaceably mounted on the piston rod covers. After a certain displacement of the piston, a portion of the outlet opens to reduce the flow through the cross-sectional area and brake the piston movement.
U.S. Pat. No. 4,363,629, which issued to Hall et al on Dec. 14, 1982, describes a hydraulic system for an outboard motor with sequentially operating tilt and trim means. The marine propulsion device comprises a transom bracket adapted to be connected to boat transom. It further includes a first pivot connecting a stern bracket to the transom bracket for pivotal movement of the stem bracket relative to the transom bracket about a first axis which is horizontal when the transom the transom bracket is mounted on a boat. A second pivot connects the swivel bracket to the stem bracket below the first pivot for pivotable movement of the swivel bracket with the stem bracket and relative to the stem bracket about a second pivot axis that is parallel to the first pivot axis. A king pin pivotally connects a propulsion unit, including a rotatably mounted propeller, to the swivel bracket for steering movement of the propulsion unit relative to the swivel bracket about a generally vertical axis and for common pivotal movement with the swivel bracket in a vertical plane about the first and second horizontal axes. A trim cylinder-piston assembly is pivotally connected to the stern bracket and to the swivel bracket. A tilt cylinder-piston assembly is pivotally connected to the transom bracket and to the stern bracket. A fluid conduit system communicates between the source of pressure fluid in each of the tilt cylinder-piston piston assembly and the trim cylinder-piston assembly. The system further includes apparatus which is operable, during reverse operation of the propulsion unit, for causing initial full extension to the trim cylinder-piston assembly, followed by extension of the tilt cylinder-piston assembly. It also causes initial full contraction of the tilt cylinder-piston assembly followed by subsequent contraction of the trim cylinder-piston assembly.
U.S. Pat. No. 4,168,800, which issued to Quick on Sep. 25, 1979, describes a combination hydraulic cylinder and shock absorber. The combined hydraulic cylinder and shock absorber is used in combination with a cantilever supported boom having a plurality of longitudinally spaced discharge nozzles thereon for a flowable material with one end of the boom being pivotally supported from a vehicle, or the like, for vertical adjustment in a vertical plane with a hydraulic cylinder enabling angular adjustment of the boom about a horizontal axis and including a structure which provides a shock absorber to dampen and cushion vertical oscillation or bouncing movement about a horizontal axis as the boom and vehicle transverse uneven terrain such as when applying liquid fertilizer or other flowable material agent to the soil.
All of the patents described above are hereby expressly incorporated by reference within the present specification.
Known hydraulic actuators with shock absorbing capability typically use at least one piston, disposed within the cylinder, which has a plurality of passages through the piston in which check valves are located. Each check valve typically uses a ball and spring combination to maintain the passages in a closed status until a preselected pressure overcomes the spring force to move the ball and open the passages. Since devices of this type are intended to react to a sudden change in pressure in one chamber of the cylinder, the effective area of each ball exposed to the pressure must be carefully sized so that the resulting force on the ball does not easily overcome the force of the spring which opposes movement of the ball toward an open position. Since a certain flow must be accommodated through the piston when sudden shock loads are experienced, a plurality of check valves must be provided so that the accumulated area of all of the passages is sufficient to allow the required flow to pass through the piston. In most known systems, five to seven passages must be provided through the piston to allow the required flow volume while also allowing the balls of the check valves to be supported in a closed position by springs of reasonable size and strength.
Since the innovative development of the hydraulic power trim and power tilt system described in U.S. Pat. Nos. 3,999,502 and 4,050,359 was originally developed, most subsequently developed cylinders with shock absorbing capability have followed the basic technique described in these patents by Mayer. As a result, most known shock absorbing trim cylinders and tilt cylinders require that numerous check valves be provided in the trim piston. As a result, each cylinder requires the expensive assembly of finely machined spheres into finely machined passages through the piston. The spheres are typically peened in place and a spring is assembled into each passage to provide a force that maintains the machined ball in a closed position to prevent the flow of hydraulic fluid through the trim piston when no shock forces exist.
The use of a plurality of check valves in prior art shock absorbing cylinders increases the cost of those devices. In addition, the balls and their respective seats must be accurately machined to provide an adequate seal so that fluid flow through the piston passages is prevented except during the occurrence of a shock loading incident. This machining, in addition to the assembly of the many components, increases the cost of the shock absorbing tilt cylinder mechanisms. The machining operation also makes the device more susceptible to contamination from debris. The metal to metal seals created by the peening process is susceptible to leakage from small contaminants or imperfections included during manufacturing at the seal face.
For the reasons described above, it would be significantly beneficial if a shock absorbing hydraulic actuator could be provided which reduces the required number of components and also reduces the highly accurate machining processes that are currently required in actuators known to those skilled in the art.